Do chromosomes only form during mitosis? Are chromosomes ever present during interphase in an animal?
Let's not get confused by terminology here. Chromosomes are always present in living animal cells from the moment of conception throughout the organism's entire life. The form that they are found in is what can change depending on the phase of the cell cycle.
Mitosis is the process by which the majority of cells separate duplicated chromosomes into two identical sets in preparation for separation into two identical daughter cells. It is part of the larger cell cycle which includes interphase, mitosis, and cytokinesis. Interphase is the resting phase of the cell. This is when the DNA actually replicates itself into two identical sets of chromosomes. But the chromosomes at this point are more loosely organized into material called chromatin, which is DNA bound to various proteins called histones. During the prophase of mitosis, the chromatin becomes more tightly coiled and the chromosomes take on a more defined shape where they appear as X-shaped and are bound together at a point called the centromere. Metaphase, anaphase, and telophase separate the duplicated chromosomes into two separate sets, and cytokinesis is the process where the two new cells separate.
So if by chromosomes you mean the tightly defined, X-shaped form of the DNA then yes, they are present during mitosis and not during the interphase cycle. But remember that the chromosomes are always present in the cell during interphase, just in a less tightly bound form.